Making sense of the universe is an ongoing struggle, and the more we look, the more we find that we need to understand. Some mysteries are nearby, like Venus’ weird “days are longer than years” rotation. Others force us to try and understand what is going halfway across the universe. 

Observations of the galaxy cluster <deep breath> SDSS J223010.47-081017.8 <exhale> uncovered a super weird splotch that looks rather like someone scraped a paint knife against the canvas of the universe. Close analysis of this splotch revealed they are essentially mirror images of one another and significantly more distant than the galaxy cluster that was being imaged. 

When we see weird-looking objects in the field of a galaxy cluster this can only mean one thing: gravitational lensing. The mass of the galaxy cluster bent light intended for another corner of the universe toward us, allowing us to see a distorted image of that distant galaxy. This is like using a funhouse mirror to look around a corner – a magnifying funhouse mirror.

Gravitational lenses aren’t unusual, but this particular system is unusual in how it produces twin images that are reversed. After a lot of head-scratching and phone calls to collaborators, it was realized this situation could be explained if the distant galaxy aligns with a dense ripple of dark matter that lens the galaxy’s light the way waves on a pool act like lenses to create bright patterns of sunlight on the bottom of a pool.

In addition to the mirrored images, the mass in the cluster also creates a third image of the background galaxy. This is the only lensed object in this system. These results were published in the Monthly Notices of the Royal Astronomical Society and led by Richard Griffiths.

More Information

Hubble press release

“Hamilton’s Object – a clumpy galaxy straddling the gravitational caustic of a galaxy cluster: constraints on dark matter clumping,” Richard E Griffiths et al., 2021 May 17, Monthly Notices of the Royal Astronomical Society